This invention relates to doppler radionavigation systems, and in particular to the frequency measurement apparatus used in such systems.
Doppler navigation systems are characterized in the use of transmitters coupled with antennas which produce a radiation pattern in space wherein the frequency of radiation varies with a selected angular coordinate. Transmitting antenna systems of this type have been described in U.S. Pat. Nos. 3,864,679, 3,864,680 and 3,845,486.
FIG. 1 is an illustration of a doppler radionavigation system in accordance with the prior art of the type to which this invention is pertinent. In the system of FIG. 1 there is provided a linear array 20 of antenna elements 22a through 22m. A commutator 24 is associated with array 20 and is designed to sequentially supply signals provided at the commutator input 26 to the antenna elements 22. As a consequence of the operation of commutator 24, when radio frequency signals are supplied to input 26 and commutator 24 is activated to sequentially connect input 26 to antenna elements 22a, 22b, etc., there appears to be a moving source of radiation along the antenna aperture formed by linear array 20. In the system illustrated in FIG. 1, commutator 24 is designed to alternatively provide signals in a first sequence starting with element 22a and ending with element 22m, or in a second sequence, starting with element 22m and ending with element 22a.
It should be recognized that antenna elements 22 may comprise columns of elements in a direction orthogonal to the array to provide radiation pattern shaping. Alternatively, other frequency coding antennas such as those described in U.S. Pat. Nos. 3,864,679 and 3,864,680 may be used in lieu of the commutated array 20.
Those familiar with this type of system will recognize that the sequential radiation from elements 22 appears to an observer in the radiation field of the antenna to be a source of radiation which is moving along the aperture of array 20. When commutator 24 is operated in its first sequence, the motion appears to be in the direction of the arrow indicated in FIG. 1 at a velocity V corresponding to the switching rate of commutator 24. When commutator 24 is operated in its second sequence the motion of the source is reversed.
According to well-known principles the moving source of radiation causes there to be an apparent frequency shift in the radiated pattern, which depends on the angular position of the observer with respect to linear array 20. The amount of frequency shift is proportional to the sign of the angle .theta. from the broadside axis 25 of array 20. In addition, the frequency variation of radiation with angle .theta. is reversed, when the apparent motion of the source of radiation is reversed, upon activation of the second sequence of commutator 24.
Array 20 may be used to provide angular position information in an aircraft microwave landing system. In such a system an aircraft receiving the radiation from array 20 may make a frequency measurement of the radiation and therefore determine its angular position with respect to array 20 and consequently with respect to a runway. By using two orthogonally positioned arrays, each similar to array 20, an aircraft may receive coded information to determine both azimuth and elevation position information with respect to a runway. An additional antenna equipped with a transponder may be used to determine range information, thereby providing a pilot with a complete set of positioning information.
In a microwave landing system the receiver which is to make a frequency measurement of the radiation from array 20 is located on the aircraft, which is naturally moving at a significant velocity with respect to array 20. The motion of the aircraft itself causes a frequency shift of which cannot easily be distinguished from the angular frequency variation F.sub.D of the radiated signal from antenna 20. In order to enable a measurement of aircraft angular position independent of aircraft velocity, a second antenna 28 is provided to radiate a reference signal, which when detected an aircraft has the same frequency shift as a result of aircraft motion, since the aircraft's relative velocity with respect to the two antennas is substantially the same. In order to prevent radiation interference between the signals radiated by antennas 20 and 28, each antenna radiates a slightly different radio frequency.
Reference antenna 28 is shown in FIG. 1 to be an array of antenna elements 30 which may be similar to elements 22 of array 20. A power divider is provided to supply wave energy from an input port to the various elements 30 in an amplitude and phase to cause antenna 28 to radiate a signal into the desired angular region of system operation. Elements 30 are therefore supplied with wave energy having amplitude and phase characteristics which result in the desired radiation pattern, including possible sidelobe suppression or pattern emphasis in a selected direction along a runway centerline.
The system shown in FIG. 1 includes an oscillator 32 which continuously operates at a carrier frequency F.sub.c. The output of oscillator 32 is provided to the input power divider of reference antenna 28 and also to mixer 34. An additional oscillator 36 operates continuously at an offset frequency F.sub.o, which is selected to be substantially less than carrier frequency F.sub.c. The output of mixer 34 comprises wave energy signals at frequencies above and below the carrier frequency by the offset frequency of oscillator 36. After appropriate filtering by filters 38 and 40 the signals are supplied alternately to commutator 24 by switches 42 and 44. When commutator 24 is operating in its first sequence during a first time interval, switch 42 is closed and the commutator is supplied with wave energy signals at a frequency higher than the carrier frequency. When commutator 24 is operating in its second sequence during a second time interval, switch 44 is closed and signals are supplied to commutator 24 at a frequency which is below the carrier frequency by the offset frequency.
In sequential dual scan operation commutator 24 is operated in its first and second sequences during alternating time intervals, and the signal provided to commutator 24 is alternated between a signal above and below the frequency of the signal supplied to reference antenna 28. This may be achieved by the apparatus of FIG. 1, whereby the signals supplied to commutator 24 are alternated between signals at frequencies above and below a fixed carrier during time intervals of duration T, as shown in FIG. 5A, or may be achieved by alternately switching signal sources between commutator 24 and reference antenna 28, as shown in FIG. 5B. In both figures the reference antenna signal frequency is shown as a solid line and the commutator signal frequency is shown as a dotted line for any time interval. A transmission may contain typically twelve such time intervals and angle measurements are made using an average value of the received signal frequency over the transmission. This averaging process is called "multiscan averaging".
FIG. 3 illustrates the center frequency relation of the signals used in the FIG. 1 system. It will be recognized by those skilled in the art that the actual radiated signals will have a bandwidth which is significantly wider than the narrow spectrum line illustrated. The bandwidth is the result of the finite time duration of the transmitted signal. For simplicity only the center frequency of the spectrum of each signal is illustrated. Reference antenna 28 radiates the carrier frequency F.sub.C. Array antenna 20 radiates during a first time interval a signal which has a frequency F.sub.C + F.sub.O. Because of the frequency space coding characteristics of array 20 an observer in space measures this frequency only if he is located on the broadside axis of array 20. This is indicated at .theta. = 0 in FIG. 3. When an observer is at an angle .theta..sub.D, he observes the radiation from array 20 at a frequency which is greater than F.sub.C + F.sub.O by an amount F.sub.D corresponding to the space frequency coding of array 20. This is indicated by F.sub.C + F.sub.O + F.sub.D in FIG. 3. This frequency of radiation is offset from the reference antenna radiation by an amount F.sub.O + F.sub.D which is called the angletone frequency and can be obtained in the receiver by detecting the beat frequency of the reference radiation and the array radiation.
When commutator 24 of array 20 is operated in its second sequential mode, during alternate time intervals, the signals supplied to the input of commutator 24 are at a frequency F.sub.C - F.sub.O. Because of the reverse sequence of radiation from antenna elements 22, the angular frequency coding of the radiation from array 20 is opposite the frequency coding during the first time interval and therefore the Doppler shift observed by a receiver at an angle .theta..sub.D is also opposite. On the left side of the spectrum diagram in FIG. 3 there are shown the received array signals observed at angle .theta. = 0 and also at another angle .theta..sub.D corresponding to the signal illustrated for the first time interval and commutator sequence to the right of FIG. 3. It will be observed that the offset from the reference carrier frequency has the same magnitude, but opposite sense.
In practical operation of a microwave landing system antenna 20 radiates a series of transmissions. These transmissions are in alternating sequences of commutator 24 and at alternate frequencies above and below reference frequency F.sub.C. The aircraft receiver 21 includes an RF stage 23 and detector 25 which detects the mixed signal of the reference carrier and the array signal both including a component .delta.F arising out of aircraft motion. The detector output is the difference or angletone frequency which corresponds to the offset frequency of oscillator 36 plus the Doppler shift frequency, which can be decoded into angular information in frequency measurement circuit 27. If there are a repeated number of transmissions alternating ones having opposite frequency offset and angular coding, the receiver can perform an averaging function to increase angular measurement accuracy. In a typical system the repetition of transmissions may be at a rate of 400 transmssions per second.
A recent report distributed by Massachusetts Institute of Technology, Lincoln Laboratory, and entitled "Doppler Azimuth Reference Errors with Out of Beam Multipath", by J. E. Evans and S. M. Sussman, dated June 30, 1976, indicated that a problem in Doppler navigation systems of the type described above could arise on account of multipath reflections from objects near a runway, such as aircraft hangars. FIG. 2 illustrates the conditions under which such a problem can arise. Array antenna 20 and reference antenna 28 are illustrated at the end of runway 60. As an aircraft 62 approaches the opposite end of runway 60 the signals radiated by antennas 20 and 28 may reach aircraft 62 by direct signal paths 66 and 68 and also by reflected signal paths 70 and 72, which are reflected from the wall of hangar 64. Under these conditions the motion of aircraft 62 with respect to antennas 20 and 28 is different than the motion of aircraft 62 with respect to hangar 64. The radiation reflected off hangar 64 is therefor received at aircraft 62 with a different Doppler shift frequency than the radiation coming directly from antennas 20 and 28. In addition, the radiation along path 70 from array antenna 20 to aircraft 62 originates at a different angle than the radiation along direct path 66. As a result this radiation has a different radiation frequency by reason of the angular variation in radiated frequency characteristic of antenna 20. FIGS. 4A and 4B illustrate the effect of the presence of the indirect or multipath radiation at the receiver in aircraft 62.
FIG. 4A illustrates the received center frequency of the signals which result from direct and multipath coded and reference signals during the first commutator sequence of antenna 20. The figure illustrates the detected angletone frequency. The signals which are received directly from coding antenna 20 and reference antenna 28 produce a detected signal with an angletone frequency at F.sub.O + F.sub.D. For reference purposes this signal C.sub.D R.sub.D is illustrated with unity amplitude. The angletone signal C.sub.D R.sub.M which is derived from the direct coded signal C.sub.D and the multipath reference signal R.sub.M is illustrated to have a magnitude .rho., which is the amplitude of the reflection coefficient of the multipath reference signal. This signal C.sub.D R.sub.M is shifted in frequency from the direct signal C.sub.D R.sub.D by an amount d which corresponds to the "scalloping frequency" or Doppler shift difference on account of the differential motion of aircraft 62 with respect to reference antenna 28 and hangar 64.
Two additional angletone signals have center frequencies which are additionally offset from the desired angletone frequency C.sub.D R.sub.D because of the frequency coding characteristics of antenna 20. These additional signals result from the mixing of the multipath coded signal C.sub.M with the direct reference signal R.sub.D to produce an angletone signal C.sub.M R.sub.D having an amplitude .rho., and from the mixing of the multipath coded signal C.sub.M with the multipath reference signal R.sub.M to produce an angletone signal C.sub.M R.sub.M with an amplitude .rho..sup.2. These signals are offset from the direct signal angletone C.sub.D R.sub.D by an amount K.theta..sub.sep, where .theta..sub.sep is the angular separation of aircraft 62 and hangar 64 when viewed from antenna 20. This angle is illustrated in FIG. 2; K is the angular frequency coding coefficient of antenna 20.
In a typical Doppler processing system the signals which result from the multipath coded signal C.sub.M do not produce a significant error in measurement of the desired angletone signal C.sub.D R.sub.D because the additional frequency offset K.theta..sub.sep resulting from the angular frequency variation of radiation from antenna 20 generally places these signals outside of the passband of the tracking filter used in the angletone signal processor. The passband of this filter is illustrated with dotted lines in FIGS. 4A and 4B.
FIG. 4B illustrates the central frequencies of the received angletone signals during the second commutator sequence of antenna 20. During this sequence the reference signal frequency is above the coded signal frequency and therefore the relative position of the multipath error signals with respect to the desired angletone signals have been interchanged. As in the spectrum illustrated in FIG. 4A only the error signal resulting from the mixing of the direct coded signal with the multipath reference signal falls within the passband of the processor tracking filter.
The presence of the additional signal within the tracking filter passband may, under some circumstances, cause a significant error in the measurement of the angletone signal as pointed out by the Evans et al. report. In particular, when the scalloping frequency, resulting from the differential Doppler shift on account of the different relative motion of aircraft 62 with respect to antenna 20 and hanger 64, has a frequency which is one half the frequency of sequential transmissions of coding antenna 20, there occurs a significant build up of multipath errors having an amplitude of the reference multipath reflection coefficient .rho..
The copending application of Richard Frazita Ser. No. 729,064 filed Oct. 4, 1976 which is assigned to the same assignee as the present invention discloses a Doppler radionavigation system which uses an alternating phase reference antenna to eliminate first order frequency measurement errors resulting from reference signal multipaths.
It is an object of the present invention to provide a signal processing apparatus which eliminates higher order errors, which occur at subharmonics of the transmisson repetition frequency in such a Doppler radionavigation system.
It is a further object of the invention to provide new and improved methods of measuring the frequency of a supplied signal.